In normal plasma torch embodiments, the two electrodes are tubular and coaxial with one in prolongation of the other and are each arranged in a support. It is necessary to provide a cooling circuit between each electrode and the support surrounding it owing to the temperatures reached. Furthermore, in order to produce the electric arc between the electrodes, means are provided to initiate said arc, these means possibly being of the type with electric discharge produced between the two electrodes or of the short-circuit type by means, for example, of the use of an auxiliary starting electrode. Torches most frequently include at least one electromagnetic coil disposed around one of the electrode supports so as to allow for moving of the catching feet of the electric arc and thus avoid any premature wear of the internal surfaces of the tubular electrodes.
As regards the means for injecting the plasmagene gas, such as air, into the internal chamber of the torch, they generally include a revolution piece coaxial to said electrodes and defining with the latter and their supports said injection chamber.
Transversal orifices are provided in the piece so as to authorize injection of the plasmagene gas derived from a feed circuit into the chamber. As the piece is directly exposed to the thermic radiation generated by the electric arc and the chemical reaction which ensues with the plasmagene gas, said piece is made of a metallic material and further comprises a cooling circuit. In order to do this, longitudinal passages for circulation of the cooling fluid are provided in the revolution piece. For example, these passages communicate on one side with an external annular groove provided in the piece into which the cooling fluid arrives, and on the other side these passages are placed in communication with the cooling circuit of the downstream electrode (with respect to circulation of the plasmagene gas). By means of this disposition, the same cooling fluid travels over the cooling circuits of the injection piece and the downstream electrode.
However, as the injection piece is metallic and accordingly electrically conducting, it is essential to provide an electrically nonconducting device so as to guarantee maximum insulation between the two electrodes. To this effect, nonconducting devices are provided between the injection piece and the upstream electrode which, in addition, may act as a thermic screen for the upstream or rear section of the torch.
Thus, one can readily understand the drawbacks generated by these plasma torches and mainly concerning, owing to the temperatures reached, the complex embodiment of the injection piece of the plasmagene gas provided with an internal cooling circuit and also the need to add, for those reasons mentioned earlier, nonconducting devices requiring an increase of the spatial requirement of plasma torches and the cost of these torches.
The Applicant has thus sought to overcome these drawbacks by carrying out on a plasma torch of the type described above various tests on the injection piece so as to study its behaviour according to the temperatures encountered.
The results of these tests have shown that the injection piece did not undergo temperatures as high as one would have imagined. These results have proved that the temperature of the cooling fluid at the outlet of the longitudinal passages was only slightly different from that recorded at the inlet of said passages. The Applicant thus deduced from this that the fresh plasmagene gas injected continuously through the orifices in the direction of the chamber constituted an effective thermically protective layer for the internal wall of the injection piece in relation to the temperature existing in the middle of the chamber, that is at the level of the electric arc.